The pathogenesis of myelofibrosis in myeloproliferative disorders (MPD) and acute myelogenous leukemia (AML) is poorly understood. The primary objective of this study is to characterize the in vitro fibroblastic colonies considered the progenitors of bone marrow fibroblasts, employing cytogenetic analyses, light and electron microscopic examination and measurements of their ability to enhance the growth of hematopoietic progenitor cells in culture. These studies will be performed serially and correlated with the clinical course. We will attempt to determine if the origin of the fibroblastic colonies in MPD and AML is hematopoietic or nonhematopoietic and whether the number, structure, cytogenetic composition and colony stimulating activity of these colonies correlate with the clinical course. The results of our studies should provide insight into the nature of myelofibrosis appearing in the MPD and AML, i.e., does this represent an integral part of the primary disease process or a nonspecific marrow stromal reaction. The in vitro culture systems employed will enable us to study the regulation and cellular interaction between hematopoietic and stromal elements in normals and patients with these hematological disorders. These studies may be of predictive value as regards the development of myelofibrosis and perhaps lead to alternate therapeutic modalities.